Hydropower facilities could unintentionally increase greenhouse gas emissions if they are not designed and sited keeping climate impacts in mind, according to a new study from the Environmental Defense Fund (EDF).
Reservoirs attached to these facilities can emit carbon dioxide and methane due to decomposing vegetation under the water, meaning that "hydropower isn’t necessarily a low-carbon energy source,” Ilissa Ocko, one of the authors of the report, said. The study examined 1,500 facilities — representing half of hydropower generation in the world — and found that more than 100 of them cause more warming than fossil fuels.
Hydropower advocates pushed back against the report, saying these facilities are not a major source of emissions and the “real story” is the industry’s role in decarbonizing the electric system to meet clean energy targets.
Many hydropower facilities compare favorably with other renewable energy sources in terms of greenhouse gas emissions, according to Ocko. However, “we need to dispel that myth that all hydro is good for the climate,” she said.
Nearly 22 gigawatts of hydropower capacity went online across the globe in 2018, with the majority sited in China, Brazil and Pakistan. The U.S. had nearly 80,000 MW of hydroelectric generation capacity in 2018, according to the Energy Information Administration, around half of which is based in Washington, California and Oregon.
Some of these facilities are carbon sinks, according to EDF’s research — but others are emitting greenhouse gases on par with fossil fuel sources. The researchers looked at the climate impacts of hydropower facilities over time and found that in the near term, they can have a large impact on warming.
Quantifying the overall cost versus benefit of hydropower isn’t easy because of the huge range in emissions produced by individual facilities, according to Ocko. While fossil fuels create emissions due to combustion, hydropower facilities create greenhouse gases through a different process: the decomposition of vegetation and organic matter submerged in reservoirs.
“It’s a biological process, but we are causing it to occur when we develop and maintain these reservoirs for hydropower,” Ocko said.
Developers of new hydropower plants can reduce emissions by keeping in mind their “power density,” which is the ratio of the size of the reservoir to the amount of electricity produced. Facilities that generate a small amount of power but are based on large reservoirs are likely to contribute a higher share of emissions. In addition, developers could avoid siting facilities in hot climates, where decomposition could produce a higher degree of methane emissions, the study said.
Existing hydro plants can mitigate carbon impacts by setting up systems to capture methane emissions, “but it’s a new emerging science and so there’s a lot more to be done to actually quantify how much you can capture and from where,” Ocko said.
Hydropower provides many climate-related benefits, a spokesman for the National Hydropower Association, LeRoy Coleman, told Utility Dive in an email. The U.S. Department of Energy (DOE) has estimated that increasing hydropower capacity by 50 gigawatts by the middle of the century could reduce greenhouse gas emissions by 5.6 billion metric tons, he said.
The EDF study is based on simplified assumptions and should not be used to influence local and global policies on hydropower, Coleman said. He added that scientists and regulators have yet to reach a consensus on determining the net emissions from hydropower generation, and that only 3% of the 90,000 dams in the U.S. are tied to a hydropower plant.
“If methane emissions are an issue, it is one for natural rivers and reservoirs and manmade reservoirs, not centered on hydropower generation itself,” he said.